Comment 1 for bug 1082100

I've just managed to reproduce this issue in a fresh Thunderbird profile, but it seems I'll need to revise my steps to reproduce. The offline support feature and the synchronize button seem to be relevant. Here are my steps to reproduce now:

1) Select a CalDAV calendar with "offline support" enabled
2) Create an event
3) Click "synchronize"
4) Restart Thunderbird
5) Try to change the start time of the event created earlier

Also, it turns out that it's not necessary to create the event in Lightning in order to trigger this bug: it's also possible to trigger it with events created elsewhere, like so:
1) Add a CalDAV calendar to Lightning with "Offline Support" enabled
2) Close Thunderbird
3) Use another calendar application (I used Evolution) to create an event on the same CalDAV calendar
4) Start Thunderbird, the new event should show up on the Lightning calendar display as well
5) Click "synchronize"
6) Restart Thunderbird
7) Try to change the start time of the event created earlier

HTH!